Faraday Rotation is justifiably a classic experiment, as it historically provided the first evidence ever that magnetism had anything whatever to do with light.  Faraday, using a special glass of his own manufacture, showed that linearly-polarized light underwent a systematic rotation of its plane of polarization, as the light travelled through the glass in the presence of a static longitudinal magnetic field.

TeachSpin’s ‘full Faraday’ Immersion introduces you to a variety of ways to detect, demonstrate, and quantify this effect.  The apparatus is simple enough, with a polarized red (or green) laser source, a solenoid into which the sample can be introduced, a downstream polarization analyzer, and a photodiode detector.

What’s special about this Immersion is the progression of detection methods, intended to teach students a series of data-acquisition techniques of increasing sophistication and sensitivity.  First qualitative demonstrations by a ‘protractor method’ unambiguously establish the existence and sign of the effect, but suffer from the small size of the detected rotations.  The use of a polarization analyzer not ‘at extinction’ but 45 degrees away from extinction, together with the use of quantitative photodiode detection, offers a better way to quantify the polarization rotation.

This Immersion will also introduce two techniques of even greater sensitivity.  The use of alternating, rather than steady, current in the solenoid can produce an a.c. signal at the photodetector, and this signal can be detected by lock-in methods to obtain remarkably higher sensitivity.

Participants will also be able to try out a new ‘pulsed current source’ that allows the (brief) application of solenoid currents that are ten-fold larger than could be used steadily.  This creates pulsed Faraday rotation, with rotation angles >45° readily (though briefly) attained.  This also creates a wonderful exercise in the oscilloscope detection of brief transient effects, a data-acquisition technique of remarkable generality.

The equipment for this experiment is commercially available at $3000 (Faraday apparatus, including two lasers and 6-A dc supply) plus $1125 (pulsed current source).  Other equipment needed includes a digital oscilloscope and (optionally) a lock-in amplifier.

Hosts and Mentors:

David van Baak and David Lee

[Bios and pictures to come.]

Please note that the Jonathan F. Reichert Foundation has established a grant program to help purchase apparatus used in Laboratory Immersions. Limitations and exclusions apply, but generally speaking the Foundation may support up to 50% of the cost of the required equipment.